Raman light scattering has been successfully used in critical care situations to continuously monitor a patient's respiratory gases. This technique is based on the effect which occurs when monochromatic light interacts with vibrational/rotational modes of gas molecules to produce scattered light which is frequency shifted from that of the incident radiation by an amount corresponding to the vibrational/rotational energies of the scattering gas molecules. If the incident light photon loses energy in the collision, it is re-emitted as scattered light with lower energy and consequently lower frequency than the incident photon. In a similar manner, if the incident photon gains energy in the collision, it is re-emitted as scattered light with higher energy and higher frequency than the incident photon. Since these energy shifts are species-specific, analysis of the various frequency components present in the Raman scattering spectrum of a sample provides chemical identification of the gases present in the scattering volume. The intensity of the various frequency components or Raman spectral lines provides quantification of the gases present, providing suitable calibrations have been made. In this manner, Raman light scattering can be employed to determine the identity and quantity of various respiratory and anesthetic gases present in a patient's breath in operating room and intensive care situations.
In addition to critical care situations, Raman light scattering gas analysis can also be used in many industrial applications such as stack gas analysis for combustion control, process control, fermentation monitoring, and pipeline gas mixture control. This analysis technique can also be extended to meet environmental monitoring needs in many areas such as escaped anesthetic agents in the operating room, air pollution, auto emissions testing and submarine atmosphere monitoring.
Systems developed for analysis of gases in critical care situations utilizing Raman scattering typically employ gas cells which contain a sample of the patient's respiratory gas to be analyzed. The gas sampling cell is located either within the resonant cavity of a laser or outside the cavity. In an intracavity system, a laser beam is directed through the resonant cavity such that it intercepts the gas within the sampling cell. Raman scattered light from the gas analysis region within the cell is collected by a collection optic and directed through one or more interference filters. The collection optics and interference filters and possibly focusing optics in turn transmit the Raman scattered light to appropriate detectors for quantitating each specific Raman signal, and thus, each specific gas comprising the respiratory sample.
Windows are commonly provided on either end of the gas sampling cell to protect surrounding optical elements and filters from contaminants which may be present in the gas sample. The windows further serve to confine the gas sample within the chamber, minimizing the volume of the sample and thus improving response time. In some systems, the gas cell windows can be oriented at brewster's angle to select and improve the transmission of a particular polarization of light passing through the sample. In this manner, optical losses in the laser beam which passes through the cell are minimized. However, the gas sample, in combination with particulates often carried with the sample, contaminates the cell windows and degrades the performance of the system. For example, this contamination may result in undesirable light scattering, and thus, the electrical power, and correspondingly, the laser current, required to maintain the laser light intensity is greatly increased. If untreated and uncorrected, the system will cease to function properly. Current respiratory gas analysis systems require replacement or cleaning of the gas cell to compensate for the accumulation of contaminants. This is generally a time-consuming process which involves not only the replacement or cleaning of the cell, but also, recalibration of the system, both at substantial expense in both time and money.